1. Field of the Invention
This invention relates to an absolute position electronic transducer, and, more particularly, to such a transducer using a plurality of inductively coupled transducer windings.
2. Description of Related Art
Various movement or position transducing systems are currently available. Most of these transducers are able to sense linear, rotary or angular movement. Optical transducers usually have a scanning unit and a glass scale having a grating applied to the glass scale. The scanning unit generally includes a light source, a condenser lens for collimating light from the light source, a scanning reticle with index gratings, and a photodetector. The scale is moved relative to the scanning unit. The lines of the scale coincide alternately with lines or spaces in the index grating to produce periodic light intensity fluctuations. The periodic intensity fluctuations are converted by the photodetector into electrical signals, which are then processed to determine position. Optical transducers can provide position measurements with very high accuracy, particularly if laser light sources are used.
Currently, to improve efficiency, most manufacturers prefer using hand-held encoders and other measurement tools on the workshop floor, rather than in contaminant-free inspection rooms. Optical transducers, however, are sensitive to contamination and are therefore impractical to use openly in most manufacturing or shop environments. Therefore, these optical transducers use expensive and sometimes unreliable environmental seals or other methods of encapsulating the optical transducer to keep dust and oils from reaching the transducer. Furthermore, the light source often requires a fairly large current. Thus, battery-powered measuring tools, for example, hand-held encoders, generally cannot use optical transducers.
Capacitive transducers draw very little current. Therefore, they are well suited for battery-powered measurement tools. Capacitive transducers use a plurality of capacitors formed by parallel plates. The transmitter plate and the receiver plates are mounted on a first member. Appropriate voltage generating and read circuits are coupled to these plates, respectively. Each of these plates form one plate of a capacitor. The other plate for each capacitor is provided on a relatively moveable member or scale, as one of many spaced-apart plates. As the scale is moved relative to the first member, the transmitter and receiver plates capacitively couple to various ones of the plates on the scale. The read circuitry detects changes in voltage in the receiver plate as the plates in the scale move relative to the transmitter and receiver plates.
Capacitive transducers, however, require a small gap between the plates on the stationary member and the plates on the scale. This small gap requires tight tolerances, resulting in increased manufacturing costs. Additionally, capacitive transducers are sensitive to contamination, particularly dielectric fluids, such as oils, and conductive fluids, such as water or coolants. Therefore, as with optical transducers, capacitive transducers require expensive and unreliable seals in many environments.
Magnetic transducers are insensitive to contamination caused by oil, water and other fluids. Magnetic transducers (e.g., Sony Magnescale encoders.TM.) employ a read head detecting magnetic fields and a ferromagnetic scale selectively magnetized with periodic, magnetic patterns. The read head senses magnetic field changes in the magnetic scale pattern as the scale is moved to determine position. However, magnetic transducers are affected by small particles, particularly ferromagnetic particles, attracted to the magnetized scale. Consequently, magnetic transducers, as with capacitive and optical transducers, must be sealed, encapsulated or otherwise protected to prohibit dust from diminishing their effectiveness.
Inductive transducers are insensitive to cutting oil, water or other fluids and are insensitive to dust, ferromagnetic particles, and so forth. Inductive transducers (e.g., INDUCTOSYN.RTM. type transducers) employ multiple windings on one member, such as a series of parallel hairpin turns repeated on a printed circuit board. The multiple windings transmit a varying magnetic field that is received by similar windings on another member. An alternating current flowing in the windings of the first member generates the varying magnetic field. The signal received by the second member varies periodically with the relative position between the two members. Therefore, the appropriate circuits are able to determine the relative position between the two members. Both members, however, are active. Therefore, both members are electrically coupled to their respective circuits.
Electrically coupling both members increases manufacturing and installation costs. Additionally, because inductive transducers require both members to be electrically coupled, inductive transducers are difficult to incorporate into hand-held devices, such as calipers. Furthermore, in the case of rotary encoders, the moving member is connected via slip rings, which increase the cost and decrease the reliability of the encoder.
Several conventional encoders attempt to provide a motion or position transducer insensitive to contaminants, yet more inexpensively manufactured than the optical, capacitive, magnetic or inductive transducers described above. U.S. Pat. No. 4,697,144 to Howbrook, U.S. Pat. No. 5,233,294 to Dreoni, and U.S. Pat. No. 4,743,786 to Ichikawa et al., and British Patent Application 2,064,125 to Thatcher show position detection devices that sense position between an inactive or unenergized member and an energized member. These transducers eliminate electrical coupling between the two moving members. However, they generally fail to provide a sufficiently refined transducer matching the high accuracy provided by the conventional transducers, such as optical or inductive encoders. They also have other drawbacks, such as limited measuring range, expensive and comparatively bulky construction, and/or inherently weak signal strength.
To provide sufficient signal strength, the inactive member is preferably ferromagnetic to produce a strong magnetic field. Alternately, the inactive member moves within a bulky structure that concentrates the magnetic field generated by the active member. Furthermore, these transducers cannot be used in a wide variety of applications, such as in low-power hand-held measurement tools, or linear, rotary, angular and other types of position transducing applications that require accuracies of at worst on the order of 10 .mu.m.
U.S. Pat. No. 4,893,077 to Auchterlonie describes an absolute position sensor employing several linear tracks of inductive transducers. Each track of this sensor has a slightly different wavelength or frequency. The circuits in the sensor analyze the phase difference between the tracks to determine the absolute position of the read head. Similar known systems employ capacitive transducers having multiple tracks of capacitive elements, such as U.S. Pat. Nos. 4,879,508 and 5,023,559 to Andermo. The absolute position sensors of Auchterlonie and Andermo, however, suffer from the traditional problems of inductive and capacitive transducers described above.
Howbrook's transducer employs several pitches of coils (each pitch representing 360.degree. of phase change) to similarly provide an absolute position using an inactive member. This transducer, however, has a limited range within which to determine the absolute position of the inactive member. Additionally, this transducer fails to provide sufficient accuracy for most applications.
Several products by HEIDENHAHN employ optical transducers having a photodetector and a scale. The scale has optical markers on it for identifying a coarse absolute position relative to the scale. Some HEIDENHAHN products, however, require positioning and identifying the markers with great accuracy. Positioning such markers with great accuracy requires costly manufacturing techniques. In any case, these optical transducers suffer from the same limitations of contamination sensitivity and power consumption described above.
U.S. Pat. No. 5,027,526 to Crane describes an optical transducer that reads a bar code pattern printed on a coiled tape. This bar code pattern is the standard interleaved 2 of 5 bar code symbol that encodes several numbers between start and stop bar code patterns. The numbers, in turn, correspond to a coarse absolute position of the tape. Circuits read the bar code symbols and converts them to numbers representing the absolute position of the tape. Clockings based on the position of a drum that coils the tape determine a fine position measurement.
This absolute transducer, however, suffers from the traditional problems of optical transducers discussed above. Furthermore, this absolute transducer is not a true absolute transducer at every position, because the transducer requires a scanning motion through a range as long as the bar code in order to derive or update an absolute position measurement. This renders it unusable for many applications.
There is thus a need for an absolute position transducer system that: 1) is insensitive to contaminants such as oil and ferromagnetic particles; 2) is suitable for a wide variety of applications, including long measuring range applications and low-power applications; 3) is accurate; 4) is relatively inexpensive to manufacture compared to the conventional transducers described above; and 5) provides an absolute position output signal. A transducing system providing at least these five benefits has until now been unavailable.